This invention relates generally to marine seismic surveying and more particularly to floatation system for use in marine seismic surveying.
In marine seismic surveying, to obtain geophysical information relating to the substrata located below the sea bottom, seismic sources, generally acoustic transmitters, adapted to produce pressure pulses or shock waves under water, are towed beneath the water surface behind a marine vessel. The shock waves propagate into the substrata beneath the sea where they are refracted and reflected back to the sea. The returning shock waves are detected by sensors (usually hydrophones) and the useful data contained in the signals produced by the sensors is processed to determine the geophysical structure of the substrata.
Air guns or gas guns are frequently used as acoustic transmitters. Usually, several air guns are placed in spaced relation to each other in a subarray. One or more air gun subarrays are towed behind a marine vessel beneath the sea surface. During operation, all air guns in a subarray are activated simultaneously to produce a desired overall pressure pulse from that subarray. The pulse characteristics, such as the frequency, bubble ratio and amplitude, of the overall pressure pulse produced by an air gun subarray is a function of the characteristics of the pressure pulses produced by the individual air guns and the physical arrangement of the air guns in that air gun subarray.
In order to repeatedly produce and transmit pressure pulses having known characteristics under water, it is important that the air gun subarray is maintained at a constant depth below the water surface and in a near straight line horizontal position. Air gun subarrays presently in use are generally more than fifty (50) feet long and weigh several hundred pounds. To tow such an air gun subarray below the water surface, it is a common practice in the art of seismic surveying to pivotly attach a single or multiple floatation devices (buoys) along the length of the air gun subarray by means of a plurality of links. The floatation device maintains the air gun subarray at or near a constant depth below the water surface when the subarray and the floatation device combination (or the seismic source system) are towed behind a vessel.
Conventional floatation systems for use in marine seismic acquisition typically consist of sealed metallic containers, sealed members having solid floatation material in separate compartments, or sealed members that communicate with one another using a series of valves and a regulated air supply. Flotation systems consisting of sealed metal containers frequently suffer from premature failure in operation because of water infiltration of the inflexible metal container. Floatation systems consisting of sealed members having solid floatation materials in separate compartments are complex and difficult to monitor in operation. Floatation systems consisting of sealed members that communicate with one another and include a regulated air supply are complex and require a regulated power supply, valving, and a controller.
The present invention is directed to overcoming one or more of the limitations of the existing floatation systems used for marine seismic energy source.